Problem: The areas of three squares are 16, 49 and 169.  What is the average (mean) of their side lengths?
Explanation: Since the areas of the three squares are 16, 49 and 169, then their side lengths are $\sqrt{16}=4$, $\sqrt{49}=7$ and $\sqrt{169}=13$, respectively.

Thus, the average of their side lengths is $$\frac{4+7+13}{3}=\boxed{8}.$$